(i) Field of the Invention
This invention relates to piezoelectric ceramics which are used for ultrasonic vibrators, ceramic filters, surface acoustic wave devices, etc. The piezoelectric ceramics are especially used for the surface acoustic wave devices.
(ii) Brief Description of the Prior Art
As the piezoelectric ceramics of this type, there have heretofore been well known the so-called PZT type piezoelectric ceramics which contain lead zirconium titanates (PbZr.sub.1-x Ti.sub.x O.sub.3) as the principal constituents and the so-called SPN type piezoelectric ceramics which contain sodium-potassium niobates (K.sub.1-x Na.sub.x NbO.sub.3) as the principal constituents. Regarding the PZT type ceramics, ones which are considerably excellent in both the piezoelectric property and the temperature characteristics have been obtained by adding third constituents such as Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3 to the principal constituents. Since, however, they are essentially ceramics of high dielectric constants (400-1,500), the input and output impedances of devices lower in high-frequency uses, which poses a problem in the impedance matching with an external circuit. In particular, the frequencies of piezoelectric transducers have recently become higher and higher, and it has become an important subject to lower the dielectric constants. On the other hand, the SPN type ceramics have the feature that the dielectric constants are small (150-200). However, they have the disadvantages that the electromechanical coupling factors are not sufficiently great and that the temperature characteristics are very inferior.
The following references are cited to show the state of the art: (i) Japanese Patent Application Publication No. 42-4023, (ii) S. Ikegami et al; The Journal of the Acoustical Society of America, vol. 50, No. 4 (1971), pp. 1060-1066.